1. Field of the Invention
The present invention relates to switching power conversions, and more particularly relates to switching power conversions capable of performing soft switching on a primary side power switch.
2. Description of the Related Art
To reduce the power dissipation, electromagnetic interference, etc. of the switching power converters, soft switching is widely adopted due to the advantage of low conduction loss of the primary side power switch.
Taking the fly-back AC-to-DC power adapter using soft switching as an example, FIG. 1 shows the circuit diagram of a typical fly-back AC-to-DC power adapter. As shown in FIG. 1, the typical fly-back AC-to-DC power adapter includes an AC plug 101, an input rectification and filtering unit 102, a main transformer and output rectification and filtering unit 103, a feedback network 104, a PWM IC 105, a VCC regulator 106, an NMOS transistor 107 and a resistor 108.
In the architecture, the AC plug 101 is used for providing an AC input.
The input rectification and filtering unit 102 is used to generate a main input voltage according to the AC input.
The main transformer and output rectification and filtering unit 103, having a primary side coupled to the main input voltage and a secondary side coupled to a combination of a diode and a capacitor, is used to convert power from the AC input to the output Vout of the adapter.
The feedback network 104 is used to generate a feedback signal VFB according to an error signal derived from a reference voltage and the output Vout.
The PWM IC 105 is used to generate a gating signal VG according to a quasi-resonant (QR) signal VA, the feedback signal VFB and a current sensing signal VCS to drive the NMOS transistor 107, wherein the gating signal VG is expected to issue a high level at the instance when the quasi-resonant (QR) signal VA is at its valley voltage to reduce the conduction loss on the NMOS transistor 107.
The VCC regulator 106 is used to generate a DC supply voltage VCC and the quasi-resonant (Q) signal VA for the operation of the PWM IC 105, wherein the resonant waveform of the quasi-resonant (QR) signal VA is proportional to the drain voltage of the NMOS transistor 107 when the NMOS transistor 107 is off, and the valley of the quasi-resonant (QR) signal VA is corresponding to the valley of the drain voltage of the NMOS transistor 107.
The NMOS transistor 107, responsive to the gating signal VG, is used to control the power conversion via the main transformer and output rectification and filtering unit 103.
The resistor 108 is used to carry the current sensing signal VCS.
Through a periodic soft switching of the NMOS transistor 107, which is driven by the gating signal VG generated from the PWM IC 105, the input power is transformed through the main transformer and output rectification and filtering unit 103 to the output with less conduction loss on the NMOS transistor 107.
However, it is not easy to turn on the primary side power switch right at the valley of the quasi-resonant voltage because the delay contributed by the power switch and the main transformer, being unknown and dependent on the application, has to be taken into account in determining the turn-on instance of the primary side power switch.
According to this problem, a prior art U.S. Pat. No. 7,426,120 B2 has proposed a switching control circuit. Please refer to FIG. 2a-2c, which shows the waveform for detecting the valley voltage and phase lock according to the prior art switching control circuit for a fly-back AC-to-DC power converter. As shown in FIG. 2a-2c, the VM is an inverted version of a voltage from an auxiliary winding, and the prior circuit needs two sets of sampling circuit and related processing circuit to accomplish the soft switching. Due to the complex circuit structure, the prior art U.S. Pat. No. 7,426,120 B2 is not robust enough in implementing power converters, so there is a need of a concise and robust solution for soft switching the primary side power switch.